Yes, Walker may have upset the club enough for them to decide now is the time to move him on. However, if Cancello is also unsettled and wants to move that would mean 2 new right backs. That wouldn't seemingly make football sense, unless they think Semedo is already playing in a Pep like system, and will adapt seemlessley.
I think Couto is recognised as an exceptional prospect in his position. I'm thinking like Jesus and Foden have been. If called on I don't think there would be issues. If anything he might make a position in the first team his own.